The present invention relates to a method for assisting a vehicle to start on a slope wherein a vehicle brake torque is changed corresponding to an estimated vehicle driving torque in consideration of a downgrade torque.
The term ‘starting on a slope’ implies starting to drive uphill and downhill as well as starting to drive on a horizontal roadway.
A starting aid is meant to assist the driver when starting to drive uphill. The purpose of starting aids when driving uphill is to simplify the complicated handling of brake, parking brake, clutch and engine. In addition, the vehicle must be prevented by all means from rolling backwards in order to avoid e.g. a collision with vehicles positioned downhill. To this end, an assistance system of this type is required to determine the driving torque transmitted by way of the clutch to the driving axle apart from the downgrade torque and the brake torque.
WO99/20921 discloses a method, which in accordance with the determined driving torque and the determined downgrade torque, influences the brake torque in such a manner that when starting to drive uphill the sum of brake torque and determined driving torque will always exceed the downgrade torque. In this arrangement, the estimation of the driving torque is evaluated directly, that means absolutely, in an observer.
The shortcoming of this method is that errors that possibly exist in the driving torque estimation will be included in the balancing operation, and a premature or too late release of the brake will occur during the starting maneuver depending on the type of error. When there are errors in determining the driving torque, the vehicle will consequently roll away in opposition to the desired driving direction (brake is released too early), or the vehicle is held in position too long and the driver is hindered when starting to drive.
Possible errors in determining the driving torque may be due to the following causes:                Mechanically and/or electrically induced loads in the vehicle are not taken into account in the model-based determination of the driving torque. These loads may be powerful electric consumers such as a seat heating, or other retrofitted accessories.        The internal combustion engine installed into the vehicle has a definitely different friction torque compared to other engines of the same construction (production variations, aging, etc.).        Additional torques (such as ISAD moment, load moment of the air-conditioning compressor, etc.) of already provided control devices are determined and made available. However, said additional torques may be faulty. For example, the load moment of the air-conditioning compressor is mostly considered only as a static maximum value in the lost torque supplied by the engine control unit.        
An object of the present invention is to disclose a more accurate method for assisting a vehicle to start and to provide a corresponding device.